1. Field of the Invention
The invention relates to a system for rotationally coupling two shafts which slide along a common axis. The coupling system according to the invention applies in particular to an automobile vehicle steering column, in which it is fitted to the intermediate column part connected to the steering box or to the upper column part connected to the steering wheel.
2. Description of the Prior Art
In the more particular case of the intermediate shaft, this has a universal joint at each end: one universal joint is connected to the motion input of the steering box and the other universal joint is connected to the top of the column. For modern automobile vehicles to be steered correctly it is necessary for the length of the intermediate shaft to be variable and to adapt to oscillations of the front power train of the vehicle caused by the profile and condition of the road surface.
The intermediate shaft must therefore first and foremost be of variable length, i.e. it must have as one function sliding of two shafts relative to each other along a common axis, which is the axis of the intermediate part. It also requires a function of transmitting rotational movement and the rotational torque needed for steering between the two shafts.
There are many sliding shaft coupling devices which transmit torque between the two shafts using splines with conjugate profiles formed on each of the two shafts. However, this type of system suffers from backlash after 150,000 km, which is the expected service life of modern automobiles. To delay the onset of this backlash the sliding of the two shafts is adjusted relatively closely during manufacture, which requires a relatively large axial force on the production line, which increases assembly time and makes assembly more difficult.
The adjustment must be accurate to enable axial movements to be absorbed correctly in operation. When torque is being transmitted the axial force is a function of the torque and axial locking of the two shafts is followed by sudden axial unlocking, which causes jerks which compromise good sliding adjustment with low backlash and the feeling of being in control.
The object of the present invention is to propose a sliding shaft coupling system which avoids the above drawbacks, i.e. a system which requires an axial force that is not related to the transmitted torque. The axial force therefore has to increase only very slightly as the transmitted torque increases. Moreover, the system coupling the two shafts must be easy to fit into existing spaces available on automobile vehicles, with a low axial fitting force, and there must not be any backlash at the end of its service life.
The invention relates to a system for coupling an inner shaft and an outer shaft which slide one within the other in the direction of a common axis, said system including rolling members which are disposed between said inner shaft and said outer shaft. Each of said rolling members is disposed in a concave part of said inner shaft and in a concave part of said outer shaft. Each of said rolling members is associated with at least one spring member which is disposed between said rolling member and the corresponding concave part of one of said shafts.
Said rolling members are arranged in a plurality of axial rows. Each axial row of rolling members is disposed in a concave part of the inner shaft in the form of an axial groove and in a concave part of an outer shaft in the form of an axial groove. One of the two concave parts in the form of an axial groove includes a back and two flanks to receive two races in the form of an axial bar. Each race is disposed between the corresponding rolling members and one of the flanks of the axial groove. At least one spring member is disposed between the back of the axial groove and each of the two races in order to press the races against the corresponding rolling members.
When no torque is transmitted, each rolling member is always in bearing engagement on the same side as said spring member(s) and on the opposite side and so that when torque is transmitted there is for each rolling member at least one bearing area on the same side as said spring member(s) and at least one bearing area on the opposite side.
Preferably, in accordance with the invention, the rolling members are balls.
In different embodiments of the invention, each ball is provided with three spring members which are disposed between the corresponding concave part and said ball. Each spring member is a spring with a bearing shoe in bearing engagement with the corresponding ball.
In one embodiment of the invention, each ball moves on two races which are pressed against said ball by the same corresponding spring member and said races are disposed between said ball and the corresponding spring member.
In another embodiment of the invention, each ball moves on two races and is associated with two spring members, each race is pressed against said ball by the corresponding spring member and each race is disposed between said ball and the corresponding spring member.
To prevent the balls rubbing against each other, the balls are disposed in at least one retaining cage.
To make the operation of the coupling device according to the invention safer, male grooves and female grooves with conjugate profiles and with some clearance are formed on said inner and outer shafts so that torque can still be transmitted between said inner shaft and said outer shaft if said balls rupture.
Various coupling system structures can be designed. In one structure, the spring member or each of said spring members is disposed in said inner shaft. In another structure, the spring member or each of said spring members is disposed in said outer shaft.
In a further structure, some spring members are disposed in said inner shaft and others springs in said outer shaft.
In one particularly well-balanced embodiment of the coupling system, the balls are disposed in three axial rows and said axial rows are at 120xc2x0 to each other in a transverse plane, the inner shaft has three axial grooves which are at 120xc2x0 to each other in said transverse plane, and the outer shaft has three axial grooves which are at 120xc2x0 to each other in said transverse plane.
In another particularly well-balanced embodiment of the coupling system, the balls are disposed in two diametrally opposed axial rows. The inner shaft has two diametrally opposed axial grooves; and the outer shaft has two diametrally opposed axial grooves.
In one complete architecture of the coupling system according to the invention, the inner shaft has three axial grooves which are at 120xc2x0 to each other in a transverse plane and a spring member in the form of an axial coil spring and two races in the form of an axial bar are disposed in each axial groove; the balls are disposed in three axial rows which are at 120xc2x0 to each other in the transverse plane and said balls are held in place relative to each other by a cage in the form of a sleeve; the assembly comprising said sleeve, said balls, said races and said axial coil springs is closed at each axial end by a retaining ring which engages in each of said axial grooves of said inner shaft and one of said retaining rings bears against a ring which is mounted in a groove in said inner shaft; and the outer shaft has three axial grooves which are at 120xc2x0 to each other in the transverse plane and which slide on said rows of balls and said axial grooves have the length required to enable the required axial sliding of said outer shaft and said inner shaft.
The coupling system according to the invention applies to the intermediate part of an automobile vehicle steering column or to the top part of an automobile vehicle steering column.
The system according to the invention for coupling two shafts along a common axis therefore has the advantage of establishing bearing contact through rolling balls at all times, which balls are always in contact, even if no torque is transmitted.
Also, the axial force required during fitting on the production line is low. The accurate adjustment of the coupling system of the invention avoids axial jerks when driving the vehicle and guarantees durability by preventing backlash in the coupling system. Also, when torque is being transmitted, the axial force does not depend on the torque. Finally, the coupling system is easy to fit into the existing space for automobile vehicle steering columns.